Internet Protocol Multimedia System (IMS) networks have the concept of both public and private user identities. Public user identities are how one phone user (or ‘subscriber’) is identified by other phone users, for example when making a call, one user provides the public user identity of the other user they want to call. Private user identities are how the user identifies themselves to the core of the network; in particular, they can for example be used for authentication. Different types of subscriber data are associated with public user identities from private user identities, for example authentication credentials can be stored at the scope of a private user identity, as opposed to a public user identity.
The IMS architecture proposes a Home Subscriber Server (HSS) component, which stores this subscriber data and exposes it over a Cx interface. Authentication credentials can be retrieved from the HSS by using a Cx Multimedia-Auth-Request/Answer flow. This flow requires both a public and a private user identity to be provided, even though the underlying data is stored only at the scope of a private user identity.